If this idea has been used or extremely similar to an already existing SCP, please feel free to tell me.

So, 150 years ago a famous gunman was shot down in a gunfight by a yet-to-be-identified person named "Magee." With his dying breath the gunman - "Deadman" Morgan - swore to someday come back from the grave and shoot down Magee. "Someday" happened to be 140 years later. Morgan rose only to find that everyone, along with the town, had disappeared leaving the graveyard in the middle of nowhere. Rusted gun in hand, Morgan roamed the US looking for the now long gone Magee becoming more and more distraught as he realized that he would never find his killer. The Foundation didn't discover Morgan until 11/16/94 when he frightened many passers by in a New York alley way asking them for assistance. By then he had heavily decomposed, being well preserved in the ground he was buried in, in the open air, nearly skeletal and falling apart. The Foundation secured him and now keep him contained in Sector-28. He has since been deemed Safe being extremely docile and wishing no harm to anyone that didn't "do him no harm."

Of course, details such as dates and site names will be expunged, but for the sake of simplicity they will be shown here and obviously, most of these background details will be revealed in additional materials. Any and all criticism is welcome.

I've actually attempted a few before and my only successful one thus far has been SCP-1063, and even then it needed a complete overhaul. I do see where you are coming from though, it will be difficult to pull off, but I really want to try this.

The idea seems solid enough, if not super-interesting, at least as you've described it. Even so, in order to exploit teh pathos, you would have to do addenda, interviews, etc. all of which generally require a good amount of experience to pull off. Wait until you have one or two others under your belt and then go for it.